Bone screws are available in a plurality of variations for different applications. Bone screws which can be secured to a bone plate or a similar implant are also known as locking screws. For locking the bone screw to the bone plate, a head of the bone screw is provided with a thread that mates with a corresponding thread on an inner surface of a plate hole.
U.S. Patent Publication Nos. 2005/0277937 and 2009/0192550 relate to a typical locking screw which is intended to be secured to a bone plate. The head of the bone screw has a spherical shape and a thread with V-shaped ridges. The thread of the head is a double lead thread which mates with an internal thread of a plate hole. The ridges of the thread provided in the plate hole have a defined angle relative to the plate, whereby the bone screw is correspondingly fixed to the bone plate at a predetermined angle.
The threaded head of a locking screw may also have a cylindrical or conical shape. U.S. Pat. No. 7,179,260 and U.S. Patent Publication No. 2007/0276386 relate to a bone plate system comprising a locking screw with such a head. The screw head is completely or partially threaded to be received in a threaded plate hole. U.S. Patent Publication No. 2005/0261688 relates to a further bone screw having a conically-tapered and threaded head. The flanks and peaks of the threaded head have a trapezoidal shape for mating with internal threads of a plate hole.
EP 0 230 678 A1 relates to an endosteal screw implant used in dentistry comprising a shaft and a conical head with a spherically shaped portion. The shaft of the screw has a thread which is cylindrically shaped and rounded on its external edges for fixing the shaft into a jaw bone.
The conventional bone fasteners for locking applications have several drawbacks. During the screwing-in operation of the bone fastener head into the implant, the thread of a head of the fastener can tilt and jam within the threaded portion of the implant hole. Thereby, the flanks and peaks of the threaded head and the threaded hole may get damaged. Moreover, splinters from the bone drilling as well as other materials like parts of human tissue can contaminate the edges and grooves of the threads, whereby the thread of the threaded head and the internal thread of the hole of the implant can jam.